The invention relates to the field of RF power detectors and controllers, and in particular to RF power detectors and controllers used in mobile handset terminals for wireless applications.
The demand for greater bandwidth for mobile systems in a highly competitive landscape has forced network providers to look for low cost ways to expand existing networks. One such system that is being used is Enhanced Data Rate for Global Evolution (EDGE). The EDGE system enables the providers to expand the existing Global System for Mobile Communications (GSM) system, the standard for 70% of the worldwide market, to offer bandwidths of up to 300 kbps. This bandwidth and capacity extension can be accomplished with a relatively small upgrade to the existing infrastructure network. The system, however, does present some technical challenges. One of these is accurate power control in the transmitter section in the handset terminals.
Accurate power control is required by many RF wireless communications standards, such as GSM, CDMA, EDGE, as disclosed, for example, in U.S. Published Patent Application No. 2003/0139153 (the disclosure of which is hereby incorporated by reference), etc. This necessarily accurate power control may be difficult to implement or achieve using open loop techniques because of variations in the gains of various components due to temperature variations, frequency dependences, power supply variations, and manufacturing tolerances. It may be expensive or impossible to calibrate for all these variables during manufacture. Closed loop control of the power or amplitude of the signal is desirable for these reasons.
Closed Loop power control for Non-Constant Envelope (NCE) modulated signals, such as CDMA or EDGE however, is complicated by the fact that the instantaneous amplitude of the signal varies during transmission. Conventional systems for constant envelope modulation waveforms may compare the measured output power level to a ramp (or power profile) signal in a feedback loop. Applying this technique to non-constant waveforms, which have amplitude variations, has the effect of wiping off the desired amplitude information these signals, creating unacceptable distortion of the modulated waveform. This technique is therefore unsuitable for NCE signals.
Closed loop power or amplitude control of NCE waveforms may be accomplished by means of a gain measurement, where a priori power level information of a modulated reference signal is used and compared with a measurement of the power level of the final output signal. This a priori information provides the necessary information regarding the amplitude variations of the original modulated signal for the feedback scheme to work properly. This method of closed loop feedback however, may be sensitive to variations in the average input power applied to the reference signal input. In many applications, the reference signal average power will exhibit these variations due to a number of factors, such as temperature, frequency and part-to-part variations, making this technique unsuitable.
There is a need, therefore, for an efficient and economical system for providing closed loop power control.